The California Oak Moth is a commonly seen insect here at the Reserve. Some years, this native moth seems to be an epidemic - causing harm to our native Oak woodlands, but this is just part of the cyle of life here at the Slough.

Moths, like butterflies, are part of the order Lepidoptera and thus have a similar life cycle. Females lay whitish eggs in clusters on twigs and leaves; the eggs turn reddish or brownish as hatching approaches. The eggs hatch into tiny black caterpillars or larvae. The mature larvae are 20–30mm long and are olive green with black and yellow longitudinal lines along the back and sides and have a reddish head. When they are ready, the larvae enter the pupa stage. The pupae are white or yellow with black markings and are found hanging from bark, leaves, and branches. When the pupa is mature, an adult oakmoth will emerge. The tannish brown adults (body length 14–20mm) are commonly seen in the late afternoons from spring through fall as they hover around the oak trees. In the Monterey Bay area, these moths are on the wing from approximately June through November.

The life-cycle of the oak moth includes two and occasionally three generations per year. In most of California there are usually two broods per year -- a long-lived winter generation and a short-lived summer cycle. The first set of eggs hatch in late fall, and the small caterpillars overwinter on the undersides of oak leaves. Mature larvae are found from May to June, and adults emerge June to July. The eggs of summer adults hatch soon after, and the cycle is completed with the emergence of adults in early fall.

Mature oak moth larvae are about an inch long.

During the larval stage, they feed mainly on Coastal Live Oak and other oaks in the genus Quercus that serve as host plant for the larvae. Young larvae skeletonize the leaf surface, while mature larvae consume entire portions of the leaf.

At the Reserve specifically that means you can see them along the Long Valley Loop, East side of the South Marsh Loop and sections of the Five Fingers Loop. Off the Reserve, Oak woodlands are common.

Periodic outbreaks occur approximately every 5 – 10 years, and in the most extreme cases lead to severe oak defoliation of some trees. During these outbreaks caterpillars are often seen suspended from cobwebby strands (perhaps using the silk as a lifeline or to anchor to twigs while consuming the underlying leaf platform). Healthy oaks appear to recover from these periodic defoliation events, but oaks that are simultaneously subjected to other stresses of drought, soil compaction, fungal infections (e.g. Sudden Oak Death Syndrome), and bacterial infections may be more seriously affected by oak moth defoliation. Biochemical and physiological defenses of oaks help to resist insect attack. This resistance, however, may vary between individuals of the same species and might explain why neighboring trees exhibit widely varying amounts of defoliation. Other potential factors which may indirectly affect susceptibility include the age and condition of an oak (whether it is diseased or drought-stressed) as well as human impacts (e.g. pruning, damage to roots associated with construction or vehicle traffic, altered drainage patterns, and so on).

Predators of the California Oakmoth include birds, spiders, true bugs, lacewing larvae, and ground beetles. These moths are also parasitized by several small wasps and are susceptible to certain insect-specific fungal infections. So, although the oak moths can periodically wreak havoc on certain trees, oaks and oak moths have been coevolving for a long time and oak moths can be viewed as one of the many conspicuous insects of the Reserve.